The present invention relates to a method of communicating data in communication systems, in particular but not exclusively in optical communication systems. The invention also relates to a communication system operating according to the method.
In conventional optical communication systems comprising arrays of interconnected nodes, information is conveyed from a first node to a second node by modulating optical radiation generated in the first node and guiding the radiation, for example along optical fibre waveguides, to the second node whereat the radiation is detected and demodulated to yield the information thereat. The modulation can be either of digital or analogue form.
When digital modulation is employed, it is conventional practice to modulate a radiation source such as a laser between two states corresponding to two mutually different laser radiation output levels. Conversely, when analogue modulation is employed, for example to convey time division multiplexed analogue speech information, the laser is modulated in a continuous manner over a range of optical radiation intensities.
When assessing the quality of optical communication in the conventional systems employing analogue modulation, it is relatively straightforward to measure signal-to-noise ratio performance at the second node. However, if the analogue modulating signal is modulated with digital data, it is extremely difficult to determine a corresponding bit error rate performance at the second node; the bit error rate does not correlate in a simple manner with signal-to-noise ratio performance. Moreover, it is also problematical to include digital overhead control information when analogue modulation is employed.
In the conventional communication systems employing digital modulation, additional digital information can be added to sending client payload data for determining bit error rate and for control purposes. Such conventional systems are operable to receive sending client payload data at the first node and arrange it into fixed length blocks of data to which overhead control data is added to provide aggregate data for transmission. Examples of such conventional systems will now be described with reference to published patent applications and granted patents.
In a published European patent application no. EP 0 663 776, there is described a method of communicating block coded digital data with associated synchronization and control data. In the method, block coded digital data is communicated with associated overhead data in a data stream having a succession of coded blocks. Each block contains N symbols wherein M of the symbols comprise information to be transmitted and the remaining N-M of the symbols comprise error correcting data. The ratio M/N comprises a first information rate. The coded blocks in the data stream are divided into a succession of frames, each frame comprising F of the coded clocks. A frame overhead symbol is added for each of the frames to provide data necessary for a receiver function such as synchronization. The addition of the frame overhead symbols effectively lowers the first information rate to a second information rate Mxe2x80x2/Nxe2x80x2 as provided in Equation 1 (Eq. 1):                               M          N                =                              (                                          M                xe2x80x2                            +              b                        )                                (                                          N                xe2x80x2                            +              b                        )                                              Eq        .                  xe2x80x83                ⁢        1            
where
b=an integer chosen to provide the second information rate at a desired value.
N is less than 2n+1, where n is the number of bits in each of the symbols. The number of coded blocks F in each frame is determined from Equation 2 (Eq. 2):                     F        =                                            M              xe2x80x2                        ⁢            P                                              (                              N                -                M                            )                        ⁢            b                                              Eq        .                  xe2x80x83                ⁢        2            
where
P=a smallest value integer that will render F an integer, P being equal to the number of overhead symbols added per frame.
A plurality of X of the frames are formed into a multiframe containing FX coded blocks and PX frame overhead symbols. X is chosen to provide enough n-bit frame overhead symbols to implement the desired receiver function.
In another published European patent application no. EP 0 540 007, there is described a method and apparatus for transmitting an information-bearing signal by:
(a) generating a plurality of block signals on the basis of the information-bearing signal;
(b) generating a plurality of parity block signals on the basis of the plural data block signals;
(c) generating a frame signal containing the plural data block signals and the parity block signals; and
(d) sending out the frame signal.
In the method, each of the data block signals includes a first block synchronizing signal indicating the start of the data block signal, a data signal containing the information signal and a first parity signal derived by encoding the data signal. Each of the parity lock signals includes a second block synchronizing signal indicating the start of the parity block signal, a second parity signal and a third parity signal. Bit signals located at same bit positions in the respective second parity signals are derived by encoding bit signals located at the same positions in the respective data signals. Bit signals located at the same bit positions in the respective third parity signals are derived by encoding the bit signals located at the same bit positions in the respective first parity signals; alternatively, the third parity signal in each parity block signal is derived by encoding the second parity signal in each parity block signal.
In an international application no. PCT/FI99/00477, there are described data transmission methods in a telecommunication system. The methods are concerned with employing xe2x80x9cpayload numberingxe2x80x9d instead of or in addition to conventional frame numbering. Data in the system is split into fixed-length data blocks or payload units. The size of a block is preferably equal to or smaller than the shortest information field in frames of the protocols used. Each protocol frame carries one or more payload units. In an optimum situation, the length of the information field in a protocol frame equals n times the length of the payload unit where n in an integer. Alternatively or additionally, the protocol frame carries payload numbers both for indicating the payload units conveyed in the protocol frame and for acknowledging the received blocks.
In a United States granted patent no. U.S. Pat. No. 5,490,142, there is described a VT group optical extension interface and VT group optical extension format method. In the method, a VT group extension format defines a transport frame for the transfer of 135 bytes, each byte comprising 8 bits, the format providing a line rate of 8 640 Mbit/s. Each frame comprises a transport overhead portion and a payload portion. The transport portion comprises 27 bytes and defines various operations, administration and maintenance functions. Moreover, the payload portion comprises 108 bytes which directly correspond to one VT group of an STS-N frame. The VT group optical extension format line rate is determined an as integer multiple m of an STS-N network element clock where m is 6 if N is 1 and m is 18 if N is 3. An optical extension interface is provided between a VTG bus and an optical extension, the interface being responsive to the provision of a multiplexed VT group payload provided on the VTG bus for providing a corresponding VT group optical extension transport frame on the optical extension, the interface being further responsive to the provision of a VT group optical extension transport frame on the optical extension for providing a multiplexed VT group payload and associated path overhead to the VTG bus.
It is conventional practice in contemporary optical communication systems where sending client data does not precisely partition into the blocks to partially fill the blocks with sending client data and then to add additional justification code after the sending client data to ensure that the blocks are completely filled. This practice is known as justification and assists to ensure, for example, satisfactory radiation spectra within the conventional systems.
The amount of justification employed is a function of the payload data that can vary from client to client. When the aggregate data is received at the second node, the overhead information is isolated and interpreted, and then the blocks of data are processed to remove the justification to yield the payload data. Thus, it is not possible to perform a bit error rate measurement for the aggregate data at the second node without completely decoding the aggregate data to isolate the payload data; such complete decoding is a complex process.
In large and complex communication systems including many thousands of nodes and employing the aforementioned digital modulation, it is often desirable to be able to monitor the aggregate data modulated onto the optical radiation at sub-nodes intermediate between the first and second nodes to determine error occurrence thereat. Such monitoring is especially useful when the first and second nodes are spatially separated by several hundred kilometres and the optical radiation is conveyed therebetween through a number of fibres and associated optical repeaters and regenerators. Determination of error rate at the sub-nodes enables the performance of specific parts of the systems to be measured, for example the quality of repeaters therein or the transmission media employed. Such measurement enables defective repeaters and fibres to be isolated and, if necessary, bypassed or replaced. The systems suffer a problem that error rate at the sub-nodes cannot easily be determined without fully decoding the aggregate data to determine bit error rate; this problem arises on account of justification being employed.
It is conventional practice for communication system operators to lease communication channels to clients on the contractual basis of bit error rate not exceeding a contract specified limit. In the case of communication systems employing analogue modulation, guaranteeing bit error rate performance is difficult to determine based on signal-to-noise measurement. Likewise, in the case of communication systems employing digital modulation with justification, bit error rate can be measured but requires complete demodulation of the aggregate data to determine bit error rate.
The inventors have appreciated that it is possible to employ an alternative method of encoding data in a communication system that addresses the aforementioned problems.
According to a first aspect of the present invention, there is provided a method of communicating data in communication systems, each system including at least one channel comprising transmitting means, receiving means and data conveying means for conveying data from the transmitting means to the receiving means, the method characterized in that it includes the steps of:
(a) combining payload data and overhead data at the transmitting means to form aggregate data thereat for transmission to the receiving means, the aggregate data being partitioned into frame-like structures in which the number of overhead data bits is in a fixed ratio relative to the number of payload data bits;
(b) transmitting the aggregate data from the transmitting means to the receiving means through the conveying means;
(c) receiving the aggregate data at the receiving means, decoding the aggregate data to isolate the overhead data from the payload data thereat, and interpreting the overhead data for controlling and managing the payload data within the system,
characterized in that the transmitting means (20) is operable to generate the aggregate data (600) at a rate which is greater than the rate of receipt of the payload data thereat by substantially a fraction (Rp+Ro)/(Rp), where Rp is the rate of receipt of the payload data at the transmitting means (20) and Ro is the rate at which the overhead data is added at the transmitting means (20) to generate the aggregate data (600).
The method provides at least one of the advantages that:
(a) the amount of timing jitter in the aggregate data propagating through the system is capable of being reduced, thereby reducing the occurrence of errors within the system; and
(b) error checking performance of the system is capable of being improved, for example bit error rate is more readily determinable from the aggregate data on account of the fixed ratio.
A frame-like structure in aggregate data comprising overhead data and payload data is defined as an arrangement of the overhead data such that the arrangement substantially repetitively occurs in the aggregate data and is operable to partition the payload data within the aggregate data.
Advantageously, depending upon application of the system operable according to the method, the fixed ratio of payload bits to overhead bits is in a range of 2:1 to 100:1. A ratio higher than 100:1 can result in synchronization problems at the receiving means, hence the aforementioned range is a practical compromise. Preferably, the fixed ratio of payload bits to overhead bits is 31:1.
Justification of payload data within aggregate data can result in complex methods being required to decode the aggregate data. The inventors have appreciated in the method of the invention that it is advantageous not to apply further justification to the received payload data when generating the aggregate data.
Conveniently, the system operable according to the method includes a plurality of channels, each channel capable of adapting to the data rate of its associated payload data, the channels thereby capable of functioning mutually asynchronously. Such asynchronous operation is important to circumvent a need for performing justification in the system, thereby providing benefits of simplified aggregate data decoding in the receiving means. In order to achieve such asynchronous operation in practice, it is desirable that each channel includes phase locked loop means for synchronizing the channel to its associated payload data.
In order to render the overhead data included in the aggregate data less vulnerable to burst interference, the overhead data and the payload data are preferably interleaved in the aggregate data.
Advantageously, the frame-like structures employed within the aggregate data comprise a plurality of frames organized into multiframes, the frames and multiframes identifiable at the receiving means by interpreting the position of overhead data within the aggregate data. The overhead data thereby provides the beneficial function of synchronization of overhead data at the receiving means. However, data block structure present within the payload data can be asynchronous to the frames and multiframes as a consequence, although this does not affect system operation.
The inventors have found in practice that each multiframe conveniently comprises in a range of 2 to 100 frames. This range is chosen as a compromise between being able to include a number of specialized functions within the overhead data but not have so many frames in each multiframe so that multiframe synchronization at the receiving means becomes problematical. In practice, it is preferable for each multiframe to comprise eight frames.
The aforementioned specialised functions advantageously incorporate a synchronization function. Thus, it is convenient that the overhead data associated with each multiframe comprises a synchronization code (FAW) for assisting the receiving means to synchronization to the multiframes. For example, the synchronization code can comprise four synchronization bytes, FAW1 to FAW4, in the overhead data. Moreover, the four synchronization bytes FAW1 to FAW4 can have, for example, binary values of 1111 0110b, 1111 0110b, 0010 1000b and 0010 1000b allocated thereto respectively.
When ensuring that multiframes in the aggregate data are not lost when communicated through the conveying means, it is desirable that the overhead data associated with each multiframe comprises an identity code (MIC) for use in identifying the multiframe. Missing multiframes are preferably identified at the receiving means by determining whether or not the identity code is incremented in a consistent manner for successive multiframes. Inconsistent incrementation is indicative of missing multiframes received at the receiving means. Conveniently, the identity code is incremented in modulo manner, for example in modulo 255; this enables a single byte to be used in the overhead data for representing the code. In practice, it is found particularly beneficial to increment the identity code in steps of a plurality of counts, for example in steps of 3 counts, for successive multiframes. In practice, inclusion of the MIC code is also found to assist with synchronization of the receiving means to the aggregate data.
Ensuring correct d.c. level stability from photodetectors used to detect the aggregate data can be problematical where a.c. coupling is employed to remove d.c. offsets from such photodetectors. In order to address this problem, the overhead data associated with each multiframe advantageously comprises balancing code (BAL) for ensuring that the overhead bytes associated with the multiframe include substantially equal numbers of 0""s and 1""s.
Moreover, ensuring that channel connections are correctly made in the system, it is desirable for the overhead data to include identity information regarding at least one of the transmitting means and the receiving means. Thus, conveniently, the overhead data associated with each multiframe comprises trail trace identification code (TTI) for use by the receiving means for confirming whether or not it is connected to its correct corresponding transmitting means.
In communication systems including a plurality of channels, failure of one or more of the channels can occasionally occur in practice. It is therefore desirable that the overhead data should be capable of invoking a channel substitution in the event of channel failure. Thus, preferably, the overhead data associated with each multiframe comprises automatic protection switching code (APS) for instructing the system to use alternative channels to convey the payload data in the event of failure of a channel within the system.
When interference occurs in the conveying means, damage to the aggregate data will often be limited to individual frames. It is therefore preferable that the overhead data associated with each multiframe comprises a bit interleaved parity (BIP) code for each frame of the multiframe, the interleaved parity code usable by the receiving means for detecting the occurrence of corruption of payload data associated with the frame. As a consequence of the number of overhead bits being in a fixed ratio relative to the number of payload bits, the BIP code provides a direct indication of bit error rate in the aggregate data; such a direct indication enables relatively simple monitors to be used for measuring bit error rate along the conveying means, for example for fault finding purposes. Thus, unlike prior art systems, the method of the invention provides a fixed density of error rate indicating code relative to client payload data irrespective of client payload data rate.
In order to assist the receiving means to synchronise correctly to the aggregate data and apply appropriate processing, for example regeneration, it is desirable that the overhead data includes an indication of the aggregate data rate at which the channel is expected to operate. Thus, advantageously, the overhead data associated with each multiframe comprises a payload type indicator (PTI) code indicative of payload data rate supplied to the transmitting means.
The method of the invention is applicable to communication systems operating at serial bit rates approaching 10 Gbits/s and greater. It is presently relatively difficult and expensive to provide logic switching devices capable of operating at such high bit rates. Therefore, it is highly desirable to convert high bit rate serial data into parallel data to ease processing tasks performed in the transmitting means and also in the receiving means. Thus, advantageously, the transmitting means is operable to receive the payload data as serial data and convert it to parallel data for combining with the overhead data to generate the aggregate data as serial data for transmission through the conveying means.
In a practical communication system, it is preferable that the conveying means comprises one or more optical fibre waveguides for conveying the aggregate data, the aggregate data being modulated onto optical radiation, for example radiation generated by a distributed feedback (DFB) laser source, which is guided from the transmitting means to the receiving means along the one or more fibre waveguides. In order to utilized fibre bandwidth to greater extent, it is desirable that a plurality of channels of the system are optically multiplexed along a single optical fibre waveguide of the conveying means.
Alternatively, for example where system portability is an important consideration, the conveying means advantageously comprises a radio link or an electrical coaxial cable for conveying the aggregate data.
According to a second aspect of the present invention, there is provided a communication system operable according to the method of the first aspect of the invention.
When implementing the system in practice, it is found advantageous for the transmitting means to incorporate an adapter unit for combining the payload data with the overhead data to generate the aggregate data, and for the receiving means to incorporate a corresponding adapter unit for separating the payload data from the overhead data. Each adapter unit beneficially includes one or more phase locked loop clock circuits for synchronization the units to data input applied thereto.
Communication systems usually have to service the requirements of several clients. Thus, conveniently, the system includes a plurality of channels operable to adapt to the rate at which they receive payload data, the channels thereby capable of operating mutually asynchronously. Such asynchronous operation enables the system to accommodate payload data being supplied from different clients at mutually different payload bit rates without a need to employ justification in the aggregate data.
In a third aspect of the present invention, there is provided a method of communicating data in communication systems, each system including a plurality of channels, each channel comprising transmitting means, receiving means and data conveying means for conveying data from the transmitting means to the receiving means, the method characterized in that it includes the steps of:
(a) synchronization at each transmitting means to its associated incoming payload data and then combining the payload data with overhead data thereat to form aggregate data for transmission to the receiving means associated with the transmitting means, the aggregate data being partitioned into frame-like structures and the channels capable of operating mutually asynchronously;
(b) transmitting the aggregate data from the transmitting means to the receiving means through the conveying means; and
(c) receiving the aggregate data at the receiving means, decoding the aggregate data to isolate the overhead data from the payload data thereat, and interpreting the overhead data for controlling and managing the payload data within the system.
In a fourth aspect of the present invention, there is provided a method of communicating data in communication systems, each system including at least one channel comprising transmitting means, receiving means and data conveying means for conveying data from the transmitting means to the receiving means, the method characterized in that it includes the steps of:
(a) combining payload data and overhead data at the transmitting means to form aggregate data thereat for transmission to the receiving means, the aggregate data being partitioned into frame-like structures in which the overhead data includes balancing codes (BAL) for substantially equalising the occurrence of 0""s and 1""s in the overhead data;
(b) transmitting the aggregate data from the transmitting means to the receiving means through the conveying means; and
(c) receiving the aggregate data at the receiving means, decoding the aggregate data to isolate the overhead data from the payload data thereat, and interpreting the overhead data for controlling and managing the payload data within the system.
In a fifth aspect of the present invention, there is provided a method of communicating data in communication systems, each system including at least one channel comprising transmitting means, receiving means and data conveying means for conveying data from the transmitting means to the receiving means, the method characterized in that it includes the steps of:
(a) combining payload data and overhead data at the transmitting means to form aggregate data thereat for transmission to the receiving means, the aggregate data being partitioned into frame-like structures comprising frames grouped into multiframes where each frame has associated therewith a bit interleaved parity code (BIP) indicative of whether or not payload data associated with the frame is corrupted;
(b) transmitting the aggregate data from the transmitting means to the receiving means through the conveying means; and
(c) receiving the aggregate data at the receiving means, decoding the aggregate data to isolate the overhead data from the payload data thereat, and interpreting the overhead data for controlling and managing the payload data within the system and determining from the interleaved parity code whether or not frames within the aggregate data are corrupted.
In a sixth aspect of the present invention, there is provided a method of communicating data in communication systems, each system including at least one channel comprising transmitting means, receiving means and data conveying means for conveying data from the transmitting means to the receiving means, the method characterized in that it includes the steps of:
(a) combining payload data and overhead data at the transmitting means to form aggregate data thereat for transmission to the receiving means, the aggregate data being partitioned into frame-like structures devoid of justification;
(b) transmitting the aggregate data from the transmitting means to the receiving means through the conveying means; and
(c) receiving the aggregate data at the receiving means, decoding the aggregate data to isolate the overhead data from the payload data thereat, and interpreting the overhead data for controlling and managing the payload data within the system.
In a seventh aspect of the present invention, there is provided a method of communicating data in communication systems, each system including at least one channel comprising transmitting means, receiving means and data conveying means for conveying data from the transmitting means to the receiving means, the method characterized in that it includes the steps of:
(a) combining payload data and overhead data at the transmitting means to form aggregate data thereat for transmission to the receiving means, the aggregate data being partitioned into frame-like structures comprising frames grouped into multiframes where each multiframe has associated therewith a multiframe identity code (MIC) which is incremented from multiframe-to-multiframe and is indicative of whether or not multiframes are missing in the aggregate data;
(b) transmitting the aggregate data from the transmitting means to the receiving means through the conveying means; and
(c) receiving the aggregate data at the receiving means, decoding the aggregate data to isolate the overhead data from the payload data thereat, and interpreting the overhead data for controlling and managing the payload data within the system and determining from the multiframe identity code whether or not multiframes within the aggregate data are missing.
In an eighth aspect of the present invention, there is provided a communication system operating according to the method of any one of the third to seventh aspects of the invention.